1. Field of the Invention
The invention relates to methods for improving the properties of photorefractive materials and to utilizing multiple layers to improve the performance, particularly the grating or image persistency of photorefractive materials.
2. Description of the Related Art
Photorefractivity is a phenomenon in which the refractive index of a material can be altered by changing the electric field within the material, such as by laser beam irradiation. The change of the refractive index is achieved by a series of steps, including: (1) charge generation by laser irradiation, (2) charge transport, resulting in the separation of positive and negative charges, (3) trapping of one type of charge (charge delocalization), (4) formation of a non-uniform internal electric field (space-charge field) as a result of charge delocalization, and (5) refractive index change induced by the non-uniform electric field. Therefore, good photorefractive properties can generally be seen in materials that combine good charge generation, good charge transport or photoconductivity, and good electro-optical activity.
Photorefractive materials have many promising applications, such as high-density optical data storage, dynamic holography, optical image processing, phase conjugated mirrors, optical computing, parallel optical logic, and pattern recognition. Originally, the photorefractive effect was found in a variety of inorganic electro-optical (EO) crystals, such as LiNbO3. In these materials, the mechanism of the refractive index modulation by the internal space-charge field is based on a linear electro-optical effect. Usually inorganic electro-optical (EO) crystals do not require biased voltage for the photorefractive behavior.
In 1990 and 1991, the first organic photorefractive crystal and polymeric photorefractive materials were discovered and reported. Such materials are disclosed, for example, in U.S. Pat. No. 5,064,264, to Ducharme et al, the contents of which are hereby incorporated by reference. Organic photorefractive materials offer many advantages over the original inorganic photorefractive crystals, such as large optical non-linearities, low dielectric constants, low cost, light weight, structural flexibility, and ease of device fabrication. Other important characteristics that may be desirable, depending on the application, include long shelf life, optical quality, and thermal stability. These kinds of active organic polymers are emerging as key materials for advanced information and telecommunication technology.
In recent years, efforts have been made to optimize the properties of organic, and particularly polymeric, photorefractive materials. As mentioned above, good photorefractive properties depend upon good charge generation, good charge transport, also known as photoconductivity, and good electro-optical activity. Various studies have been performed to examine the selection and combination of the components that give rise to each of these features. The photoconductive capability is frequently provided by incorporating materials containing carbazole groups. Phenyl amine groups can also be used for the charge transport part of the material.
Particularly, several new organic photorefractive compositions which have better photorefractive performances, such as high diffraction efficiency, fast response time, and long phase stabilities, have been developed. For examples, see U.S. Pat. Nos. 6,809,156, 6,653,421, 6,646,107, 6,610,809 and U.S. Patent Application Publication No. 2004/0077794 (Nitto Denko Technical), all of which are hereby incorporated by reference. These patents and patent applications disclose methodologies and materials to make triphenyl diamine (TPD) type photorefractive compositions which show very fast response times and good gain coefficients.
Typically, a high biased voltage can be applied to photorefractive materials in order to obtain good photorefractive behavior. While using a high biased voltage can result in a longer grating persistency, the use of a high voltage in the photorefractive material can cause the photorefractive grating to disappear almost immediately after stopping the applied biased voltage. Therefore, there is a strong need to improve grating holding persistency, even if the biased voltage is stopped and no voltage is being applied.